What to do about a flashing engine light on Nissan largo?

Go have a mechanic run a diagnostic test on it if your Largo has a computer system on it. If it doesn't, you'll still have to take it to the shop, but it will cost more to take the engine apart and find out what's wrong.
A flashing check engine light usually means you are about to destroy the catalytic converter(s) if you continue to drive the vehicle.